Technical Field
This disclosure relates to a bearing diagnostic device to sense failures in a rolling bearing used for pivotally supporting a rotation shaft in a machine tool.
Related Art
A rotation shaft device, which includes a rotation shaft such as a main spindle and is disposed in a machine tool, includes a rolling bearing (hereinafter simply referred to as a “bearing”) used to pivotally support the rotation shaft. The rolling bearing includes an inner race, an outer race, a plurality of rolling elements (such as balls and rollers), which are disposed between both races, and a retainer to keep an interval of the rolling elements. The inner race integrally rotates with the rotation shaft, and the outer race is embedded into a main spindle housing or a similar member and is secured.
Failures that occur in this bearing include a lubrication failure, an entrance of a foreign matter, a friction, an excessive load, and a similar failure. These failures cause a rotation failure and a burn-out, resulting in obstructing a normal operation of the machine tool. Therefore, it is preferable to diagnose the bearing for failure.
Japanese Patent No. 4091885 (hereinafter referred to as Patent document 1) discloses a method for diagnosing a failure in a bearing. In the method, a temperature and a stop time of a rotating device having the bearing are monitored and the stop time is compared with a reference stop time preset corresponding to a temperature so as to determine a presence/absence of a failure in the bearing. Japanese Unexamined Patent Application Publication No. 2009-20090 (hereinafter referred to as Patent document 2) discloses the following. Sounds and vibrations during operation are detected for frequency analysis. Through a comparison with a frequency spectrum level of a preset abnormal frequency, a determination for failure is performed. Furthermore, Japanese Published Examined Application 6-65189 (hereinafter referred to as Patent document 3) discloses the following. A frictional torque is calculated from a change in the count of rotations during an inertial rotation of a shaft and the frictional torque is compared with a reference value so as to determine the presence of a failure in the bearing.
With the disclosure of Patent document 1, in the case where a temperature rise in the bearing is monitored at the main spindle of the machine tool, especially a machining center, there is no choice but to install a temperature sensor to a structure near the bearing. This only results in indirect temperature measurement, thereby failing to observe a rapid change in the bearing temperature. Additionally, the accuracy of observation depends on performance of the temperature sensor itself. Therefore, there is a possibility that a failure in the bearing is not noticed, resulting in a damage of the machine.
With the disclosure of Patent document 2, a bearing failure is accurately sensable through the analysis of sounds and vibrations. However, this requires expensive measuring devices such as a vibration sensor and a sound sensor, leading to a cost increase.
With the disclosure of Patent document 3, a rate of the change in the count of rotations during the inertial rotation of the shaft is converted to obtain the frictional torque, therefore, it cannot be said that the frictional torque is accurate. In addition, the calculated frictional torque is simply compared with the reference value so as to determine the presence of the failure in the bearing, therefore, it cannot be said that accuracy of determination is high.
An object of this disclosure is to provide a bearing diagnostic device for a machine tool that can accurately diagnose a presence of a failure in a bearing at an early phase at which a damage of a machine is avoided inexpensively.